3.5 Salivary protein-encoding genes and other gall formation
associated genes
S. chinensis can induce the formation of closed galls on host
plants. Previous studies have reported that gall induction is highly
species-specific, and that galling insects deliver effectors into plant
tissues, resulting in gall formation (Yang et al., 2018). The gall midgeMayetiola destructor can inject effector proteins into tissues
via its saliva during feeding, leading to the conversion of a whole
wheat seedling into a gall (Wang et al., 2018; Aljbory et al., 2020). A
novel family of insect secreted proteins named BICYCLE have been
identified in Hormaphis cornu , which induce gall formation on the
leaves of witch hazel, Hamamelis virginiana (Korgaonkar et al.,
2021). BICYCLE may regulate numerous aspects of gall development,
due to their abundant expression in salivary glands specifically in gall
aphids. S. chinensis feeds on
host leaves where it presumably injects saliva into host leaf cells,
resulting in gall formation. A total of 141 proteins have been
identified from its salivary glands by LC-MS/MS analysis (Yang et al.,
2018). In comparison with salivary proteins from 10 other free-living
Hemipterans, the presence of a high proportion of proteins with binding
activity is noticeable, including DNA-, protein-, ATP-, and iron-binding
proteins. These proteins may be involved in gall formation. In this
study, we did not identify any BICYCLE protein in the salivary glands ofS. chinensis , suggesting the different mechanisms of gall
induction between S. chinensis and H. cornu . As
demonstrated by RNA-Seq analysis, transcripts corresponding to 35 genes
(Sc.chr03.1184- Sc.chr10.506) that encoded salivary gland proteins
exhibited high expression levels in the gall forming fundatrix stage
(Figure S5). These salivary proteins were potentially related to the
interaction between insects and host plants. According to their
predicted functions, these genes can be divided into several categories,
including detoxification, signal transduction, secreted protein
metabolism, energy metabolism,
basic biological processes and movement (Table S10). The largest number
of genes related to detoxification may be related to defense inhibition
in host plants. On the other hand, gene belonging to movement and energy
metabolism categories may be associated with the contraction of salivary
gland muscle and the supply of energy for salivation.